Closes #3767

Currently operators in most themes look like normal text because
cm-operator doesn't usually get special styling. This patch highlights
word operators (in/and/or/not) to make them stand out better.

Added: filter, endfilter, elif, endblocktrans, endspaceless

 - Add string recognition
 - Include (most probably) all built-in template tags
 - Return null as fallback
 - Operator highlighting
 - Change demo theme to `mdn-like` which sports richer highlighting.
 - Highlight comments
 - Define `blockCommentStart` and `blockCommentEnd`

Bumping them for every release just adds too much git
noise and history blowup.

Added django.js which implements the following modes

  - django (text/x-django)
  - django:inner